This invention relates to the field of logic circuits, more particularly in the area of output driver circuits using bipolar transistors.
Present push-pull output drive stages of bipolar transistor logic circuits comprise a dual transistor arrangement wherein a first transistor, which is called an upper transistor, is disposed between a DC voltage supply and an output load, and a second transistor, which is called a lower transistor, is disposed between the output load and ground. In operation a high output voltage is realized at the output terminal by turning on the upper transistor and turning off the lower transistor, and a low output voltage is realized by turning off the upper transistor and turning on the lower transistor.
In order to be able to supply large amounts of current to the load in as short a time as possible to thereby provide fast switching speeds, these output driver transistors have relatively large geometries as compared to other transistors in the logic circuit. However these large geometries give rise to problems during the turn-off of these transistors. The large geometries result in higher amount of base charge in the transistors during conduction which must be removed before the transistor will turn off, and also result in higher collector to base capacitance which can AC couple collector current into the base to turn on the transistor.
The collector to base capacitance problem is especially acute when a low capacitive load is connected to the output terminal. Since the amount of current passing across the collector to base capacitance is proportional to the rate of change of the collector to base voltage, a low capacitance load will allow the output to rise rapidly which in turn will place a fast transient voltage on the collector of the lower transistor. Thus a large amount of current is coupled into the base of the transistor while it is coming out of conduction. This current into the base of the lower output transistor will tend to turn the transistor back on so that for a period of time both the upper and lower output driver transistors are carrying current directly from the power supply through the two transistors to ground. As a result there may be current spikes drawn from the power supply which in turn waste power and introduce noise into the rest of the system. Moreover a discontinuity may occur on the output voltage which can cause the following circuits to change states twice during the single transition of the drive circuit. Finally a current spike through the transistors may cause the transistors to heat up which in turn tends to increase the Beta of the devices which causes more collector current to flow to in turn cause more heating. Under certain conditions the circuit can enter thermal runaway and destroy itself. This thermal destruction is a cause of reliability problems with present circuits.
In the past a relatively lower resistance at the base of the lower output driver transistor has been used to drain off the charge in the base of the transistor and to provide a path for the parasitic base current from the collector to base capacitance. However the lower limit of this resistance is established by other circuit parameters such as the specified DC current which can be drawn from the power supply while the output is in the low state, the increased current sourcing requirements of the previous driver transistor, and the problems associated with the larger geometries for that device. Another approach to the solution of this problem utilizes a separate DC coupled circuit which provides a lower impedance at the base of the lower output driver transistor during transitions from low to high of the output voltage. However this circuit has limited current sinking capabilities and also requires additional DC power for the added circuit. Also, to meet the general specifications of the TTL logic family, the DC coupled circuit is complex and requires an appreciable portion of an integrated circuit chip area.
Therefore it can be appreciated that a relatively simple circuit which would provide a low impedance at the base of a lower output driver transistor while the output is changing from the low state to the high state is highly desirable.
Accordingly, it is an object of this invention to provide a means of improving the rise time of a bipolar transistor logic circuit under low capacitive load conditions.
It is also an object of this invention to provide a means for preventing a multiple transition indication to the following circuitry during a single transition of the driver circuit.
It is still another object of this invention to provide a means for improving the speed power product in a bipolar transistor logic circuit.
It is still another object to provide a circuit to improve the reliability of bipolar transistor logic circuits under low capacitive load conditions.
It is another object of this invention to provide a circuit which places a low impedance at the base of the lower output driver circuit coming out of conduction and which does not use any quiescent DC power.